1. Field of the Invention
The present invention relates to a turbo-molecular pump for evacuating gas by using a high speed rotor.
2. Description of the Related Art
An example of a conventional turbo-molecular pump is shown in FIG. 13. The pump comprises a cylindrical pump casing 14 housing a vane pumping section L1 and a groove pumping section L2 which are comprised of a rotor (rotation member) R and a stator (stationary member) S. The bottom portion of the pump casing 14 is covered by a base section 15 which is provided with an exhaust port 15a. The top portion of the pump casing 14 is provided with a flange section 14a for coupling the pump to an apparatus or a piping to be evacuated. The stator S comprises a stator cylinder section 16, fixed sections of the vane pumping section L1 and the groove pumping section L2.
The rotor R comprises a rotor cylinder section 12 attached to a main shaft 10, which is inserted into the stator cylinder section 16. Between the main shaft 10 and the stator cylinder section 16 are constructed a drive motor 18, an upper radial bearing 20 and a lower radial bearing 22 disposed on the upper and lower sides of the drive motor 18 respectively. Under the main shaft 10, there is an axial bearing 24 having a target disk 24a at the bottom end of the main shaft 10 and upper and lower electromagnets 24b on the stator side. In this; configuration, high speed rotation of the rotor R is supported by a five coordinate active control system.
Rotor vanes 30 are provided integrally with the upper external surface of the rotor cylinder section 12 to form an impeller, and on the inside of the casing 14, stator vanes 32 are provided in such a way to alternately interweave with the rotor vanes 30. These vane members constitute the vane pumping section L1 which carries out gas evacuation by cooperative action of the high speed rotor vanes 30 and the stator vanes 32. Below the vane pumping section L1, the groove pumping section L2 is provided. The groove pumping section L2 is comprised by a spiral groove section 34 having spiral grooves 34a on the outer surface of the bottom end of the rotor cylinder section 12, and a spiral groove section spacer 36 surrounding the spiral groove section 34 of the stator S. The gas evacuation action of the groove pumping section L2 is due to the dragging effect of the spiral grooves 34a against gases.
By providing the groove pumping section L2 downstream of the vane pumping section L1, a wide-range turbo-molecular pump can be constructed so as to enable evacuation over a wide range of gas flow rates using one pumping unit. In this example, the spiral grooves of the groove pumping section L2 are provided on the rotor side of the pump structure, but some pumps have the spiral grooves formed on the stator side of the pump structure.
Such turbo-molecular pumps are assembled as follows. Firstly, the groove pumping section spacer 36 is attached by coupling the lower surface of the step 36a to the protruded ring section 15b formed on the base section 15. Next, the rotor R is fixed in some position, and the stator vanes 32, which are normally split into two half sections, are clamped around to interweave between the rotor vanes 30. This is followed by placing a stator vane spacer 38, having steps on its top and bottom regions, on top of the clamped rotor vane 30. This assembling step is repeated for each rotor vane 30 to complete the assembly of the stator vanes 32 around the rotor R.
Lastly, the pump casing 14 is attached by sliding it around the layered stator vane structure and fixing the flange 14b to the base of the stator S by fasteners such as bolts, thereby pressing the top stator vane spacer 38 firmly against the stepped surface 14c on the inside surface of the casing 14 and binding the entire layered assembly and the groove pumping section spacer 36. It can be understood from this assembly structure that the peripheries of each of the stator vanes 32 are pressed together by stator vane spacers 38 located above and below, and similarly the groove pumping section spacer 36 is pressed down by the lowermost stator vane 32, stator vane spacer 38 and the protrusion section 15b of the base section 15, so that the axially applied pressing force prevents induced rotation of the stator vanes 32 and the groove pumping section spacer 36 with the rotor R in the circumferential direction.
Also, though not shown in the drawing, sometimes the groove pumping section spacer 36 is fastened to the stator cylinder section 16 of the stator S by bolts to assure the fixation.
In such turbo-molecular pumps, operational difficulties are sometimes encountered, such as abnormal rotation caused by the eccentricity of rotor R, and they may be accompanied by some damaging of the rotor vanes 30. In such a case, the stator structure can also be subjected to significant circumferential or radial force by the rotor R and its debris, which may impact on not only the stator vanes 32 but the stator vane spacers 38 and the groove pumping section spacer 36.
These abnormal operating conditions can cause not only deformation of the stator vanes 32 and spacers 36, 38, but can cause fracture of casing 14 and stator cylinder section 16, or damage to their joints or severing of vacuum connections attached to the pump. Such damage and severing to any parts of the stator S cause breakage of vacuum in the whole processing system connected to and evacuated by the pump not only to damage the system facilities and in-process goods, but also to lead to accidental release of gases in the system to outside environment.